1. Field of the Invention
This invention relates generally to the field of optical fiber manufacture and, more particularly, to a method and apparatus for high rate processing of pure or doped silica tubes or fiber preforms for further processing into optical fibers.
2. Description of the Related Art
There are a variety of known apparatus and methods for forming synthetic silica tubes or preforms with a given cross sectional profile of their index of refraction. The methods include the Plasma Outside Vapor Deposition (POVD) process described by Applicants"" patent application Ser. Nos. 09/588,312 (xe2x80x9cthe ""312 applicationxe2x80x9d) and 08/994,970 (xe2x80x9cthe ""970 applicationxe2x80x9d), both of which are hereby incorporated by reference. The method described by the ""312 application deposits one or more layers at a first deposition speed followed by depositing a layer at a second deposition speed that is lower than the first. As described by the ""312 application, the first deposition speed is sufficiently high that the deposited glass is not heated to the consolidation temperature. Deposition at the lower second speed, however, consolidates both the instant deposited layer and the unconsolidated layers deposited previously at the first, higher, speed. As described in the ""312 application, the first deposition speed can be 2.5 times faster than the second, or deposition/consolidation speed. The multi-rate processing procedure described in the ""312 application thus attains up to double the deposition rate over that obtained in the prior art.
The present inventors, though, have identified a still further need for improving the processing rate attained with the Plasma Outside Vapor Deposition process, including the process carried out by the method and apparatus of the ""312 application. More particularly, the present inventors have identified a shortcoming with the existing art, which is that a low deposition rate is obtained when the target is at its smaller diameter, i.e., during the early stages of deposition. Further, the present inventors have identified a need for improving the utilization of material over the utilization attained by the existing deposition processes.
An object of the present invention is to solve the above-identified shortcoming of the prior art""s deposition rate. Another object is to improve the utilization of deposition material in the manufacturing of preforms.
A general method according to the present invention for achieving these and other objectives utilizes one or more first diameter plasma torches for depositing silica onto one or more targets to form a first intermediate preform having a first diameter, followed by an arrangement of one or more second diameter plasma torches for depositing silica onto the first intermediate preforms to form a succeeding intermediate preform or a final preform.
A first example method according to the invention comprises a first step of simultaneously depositing glass on a plurality of cylindrical targets to form a plurality of first intermediate preforms having a first diameter, using a first plasma torch, and a second step of simultaneously depositing glass on the plurality of first intermediate preforms to form a plurality of preforms, using a second plasma torch, wherein the first plasma torch has a first coil diameter and the second plasma torch has a second coil diameter, the first coil diameter being greater than the second coil diameter.
A further embodiment of this invention is a method comprising a first step of simultaneously depositing glass on a plurality of cylindrical targets to form a plurality of first intermediate preforms, using a first plasma torch, a second step of simultaneously depositing glass on the plurality of first intermediate preforms to form a plurality of second intermediate preforms, using a second plasma torch, and a third step of simultaneously depositing glass on the plurality of second intermediate preforms to form a plurality of preforms, wherein the first plasma torch has a first coil diameter, the second plasma torch has a second coil diameter, and the third plasma torch has a third coil diameter, the third coil diameter being larger than the second coil diameter and the second coil diameter being larger than the first coil diameter.